Gernreich immigrated to the United States in 1938 and, from 1942 to 1948, was a dancer and costume designer for the Lester Horton Modern Dance Troupe. From 1951 to 1959 he worked as a designer for a Los Angeles boutique. In 1960 Rudi Gernreich Inc. was formed, but he continued to design knitwear and swimsuits for other manufacturers.

Gernreich became interested in developing nonrestrictive and contemporary clothing for women. His innovative designs were intended as an alternative to the conservative styles of the then-dominant Parisian fashion houses. In 1964 he designed a topless swimsuit (“monokini”) that gained him worldwide notoriety. The unisex look, invisible undergarments, transparent tops, miniskirts, knit tank suits, and brightly coloured stockings were his trademarks.